#include <iostream>
#include <math.h>


int main() {
    int n = 0;
    std::cin >> n;
    int* x = (int*) std::malloc(n*sizeof(int));
    int* y = (int*) std::malloc(n*sizeof(int));
    for (int i = 0; i < n; i++) {
        std::cin >> x[i] >> y[i];
    }

    float square = 0;
    for (int i = 0; i < n-1; i++) {
        square+=x[i]*y[i+1] - x[i+1]*y[i];
    }
    square+=x[n-1]*y[0]-x[0]*y[n-1];
    square *= 1/2.;
    std::cout << fabs(square) << std::endl;
    std::free(x);
    std::free(y);
    return 0;
}